The present invention relates generally to machine tools and, more specifically, to electric discharge machining technology which employs electric discharge machining fluid as an insulating and cleansing medium in the production of metal dies such as forging dies. In the present invention, the electric discharge machining fluid is also used as a dust entrapment medium and flushing fluid during the machining of graphite electrodes which are used in the electric discharge machining process.
It is well known that one method for creation of dies such as forging dies, made from various alloys is the process of electric discharge machining (hereafter referred to as "EDM") using machinery designed for that purpose. In this process, a graphite electrode or block having a surface with a "positive" pattern in the form of the desired shape of a part, and a surface of a metal die block to be machined, are placed in an EDM bath and separated a predetermined distance known as the spark gap. The EDM bath consists of a tank filled with EDM fluid. EDM fluid is a trade name which identifies a fluid specially formulated for use in the EDM process. In addition to several other particular attributes well known to practitioners in the EDM art, the EDM fluid does not contain water, and is nonvolatile and nonconductive.
A voltage differential is then applied between the graphite electrode and the metal block. The voltage differential will cause opposing surfaces of the graphite electrode and metal block to spark. This sparking action will cause the surface of the metal block to erode until it assumes a shape which is the "negative" of the positive pattern on the graphite electrode. After some minor machining, the metal block can then be used as a die in various manufacturing applications such as in a metal forging operation or plastic injection molding.
During the EDM process, in addition to the metal particles caused by the erosion of the surface of the metal block, there is also a partial erosion of the surface of the graphite electrode which produces graphite particles. For the EDM to function properly it is necessary to remove any of the metal and graphite particles generated during the process. Therefore, EDM apparatuses normally include dielectric fluid pumping and filtering systems which, during the EDM process, continuously take EDM fluid from the EDM bath, filter the fluid and either circulate the cleansed fluid through the bath or use the fluid to flush the spark gap. In most such EDM devices, the fluid filtering and flushing systems operate continuously during machining.
After a metal die has been machined by the EDM process, the positive pattern in the graphite electrode will need to be remachined to the desired shape to compensate for the erosion. A numerically controlled milling machine is typically used for this machining. During the machining or remachining of the graphite block patterns, a substantial amount of graphite dust and particles are generated. This dust can cause surrounding electronic apparatuses to short out and may also pose a health hazard to the operator of the machining device.
There are several known methods for dealing with the generated dust. One method involves an industrial vacuum cleaner having a suction nozzle disposed in close proximity to the working area. However, this suction nozzle and the accompanying hoses require frequent adjustment for different workpieces and interferes with the changing of the tooling, whether by the operator or by the numerically controlled milling machine.
Another method for dealing with the production of dust is the use of transparent covering hoods. However, to be effective, these hoods must cover the entire working area of the numerically controlled milling machine, which reduces their effectiveness in achieving satisfactory dust control. In addition, these hoods tend to obstruct the vision of the operator monitoring the machining operation.
A third method of dealing with the dust is through the use of a dust containment device, as disclosed in U.S. Pat. No. 4,798,505. The device uses cutting or cooling fluid, for example, of the type employed in the machining process, to create a fluid "curtain" about the workpiece. Thus, the space around the workpiece is sealed, and the dust particles are entrapped, by the curtain from the surrounding area. The use of coolant fluid to entrap particles, however, requires a separate filter system to continuously clean the graphite dust and particles from the coolant system before it is reintroduced back into the machining process.
Another disadvantage of these filtering systems is that they typically do not remove all the material particles from the coolant fluid. Any remaining unfiltered particles eventually clog up the distribution system for the fluid curtain, causing holes in the curtain, and reducing the curtain's effectiveness in dust suppression. In addition, the use of a cutting or cooling fluid as a dust extraction curtain will contaminate the surface of the graphite electrode in the EDM process. All presence of the contaminating fluid must be removed before the graphite electrode can be used in the EDM process.
Yet another disadvantage of the above-described curtain forming device is the use of fixed sloping nozzles to divert a portion of the cutting fluid onto the tool. Due to the fixed nature of the nozzles, the flow direction of the fluid emitted from these nozzles cannot be altered to compensate for the various shaped workpieces or milling tools. Finally, because the nozzles obtain the fluid from the chamber which forms the curtain, the nozzles cannot be utilized unless the curtain is operating.